The first problem to be solved in this invention is to improve the reliability and minimize the size of the refrigerator by removing a piston or a displacer which is a moving part in a low temperature portion. In the two stage type Gifford-MacMahon (G-M) refrigerator or the Stirling refrigerator, a sliding seal is used at a low temperature. At low temperatures, elastic materials such as rubber are hardened and can not be used. In order to closely contact an outer periphery of the seal with an inner surface of a cylinder, high precision processing are needed and cost very much. Further, since at low temperatures lubrication oil or grease can not be used, the seal has to be replaced frequently due to wear. Therefore, in a refrigerator having an expander (such as a Claude cycle machine), the seal is not provided in a low temperature portion, but the seal is provided in room temperature portion by using a long piston. However, in this case, in order to decrease heat inflow due to heat conduction through the piston and another heat inflow (shattle loss) due to a difference between temperature distributions in the piston and the cylinder during osciIlating mortion of the piston, the length of the piston should be increased. This prohibits minimizing the refrigerator.
As an attempt to remove the displacer or the piston in the low temperature portion, a pulse tube refrigerator has been proposed. This refrigerator dose not have moving parts in a low temperature portion, but there is a problem to be solved such that degradation of performance of a regenerator at low temperatures should be improved to decrease an attained temperature. This is the second problem that this invention intends to solve. The degradation of parformance is caused because heat capacity of the regenerator becomes smaller than that of helium gas. Recently, it has been reported that the G-M refrigerator or the Starling refrigerator reached to temperatures below 4 K by using magnetic materials having large specific heat capacities at low temperatures as regenerator matrixes. However, the degradation of performance of the regenerator at low temperatures can not be solved in the pulse tube refrigerator.